diff --git a/yants.nix b/yants.nix
index 3dfebfcf1..a00d08439 100644
--- a/yants.nix
+++ b/yants.nix
@@ -35,7 +35,8 @@ with builtins; let
       type = def."${field}";
       checked = type.check fieldVal;
   in if checked then (current && true)
-     else (throw "Field ${field} is of type ${typeOf fieldVal}, but expected ${type.name}");
+     else if isNull fieldVal then (throw "Missing required ${type.name} field '${field}'")
+          else  (throw "Field ${field} is of type ${typeOf fieldVal}, but expected ${type.name}");
 
   checkExtraneous = name: def: present:
   if (length present) == 0 then true
diff --git a/z-structs.png b/z-structs.png
index f46e15ade..fcbcf6415 100644
Binary files a/z-structs.png and b/z-structs.png differ